The present invention generally relates to a device for sensing acoustic signals and converting them to corresponding electrical signals and vice versa. The invention more particularly relates to a hydrophone which uses a sheet of a piezoelectric polymer which is polarized and is provided with the leads to pick off the voltage output generated as a result of an impinging acoustic pressure wave.
Piezoelectric polymer technology for using microphones and hydrophones is in its infancy. To use such a material for a hydrophone in acoustic line arrays, the hydrostatic mode using the piezoelectric constant d.sub.h rather than the more sensitive d.sub.31 is preferred. It has been found that the open-circuit voltage sensitivity of a polymer hydrophone, using the hydrostatic mode, is inherently very low. Even worse, we must deal with the resultant sensitivity, reduced by the input capacitance of the amplifier. If this input capacitance is much smaller than the effective capacitance of the polymer hydrophone, the resultant sensitivity is approximately proportional to the ratio of the piezoelectric constant, d.sub.h, of the polymer to the square root of its resultant capacitance C.sub.s which is obtained by summing the capacitances of its "n" component strips connected in series, i.e., in this case the sensitivity is approximately proportional to the product of n and d.sub.h. Since the original capacitance C.sub.o of the polymer sheet is normally much larger than the input capacitance of the amplifier, it is possible to trade off a lower value of the effective capacitance C.sub. s or C.sub.o /n.sup.2 for a higher effective sensitivity of the polymer hydrophone. An optimum occurs when the input capacitance of the amplifier is approximately equal to C.sub.s, the effective capacitance of the polymer sheet. This is so because the resultant sensitivity S is exactly given by ##EQU1## where n is the number of strips into which the polymer sheet has been divided; d.sub.h is the piezoelectric constant; A is the area of the polymer; C.sub.o is the capacitance of the original polymer sheet before it has been divided (i.e., n=1); and C.sub.in is the input capacitance of the amplifier. It should be remembered that C.sub.o /n.sup.2 =C.sub.s, the effective capacitance. One way to increase sensitivity is to connect a number of polymer strips electrically in series and then cement the strips into a composite thick strip. The thick strip so made increases the sensitivity depending upon the number of strips used (at the expense of the capacitance value, which is reduced). However, it is difficult to make a usable composite thick strip because of the problem of entrapped air bubbles. It is thus desirable to have a hydrophone which uses a single flexible piezoelectric polymer sheet with many strips electrically connected in series, without the necessity of compositing a thick strip.